1. Field of the Invention
The present invention relates, in general, to the preparation of barium titanate powder for use in ferroelectrics and piezoelectrics and, more particularly, to the preparation of barium titanate powder through oxalate synthesis.
2. Description of the Prior Art
Comparable to ferrite in the materiology of electroceramics, barium titanate (BaTiO3) showing ferroelectric characteristics finds a broad spectrum of applications in multilayer ceramic capacitors, static characteristic thermistors (PTC), piezoelectrics, etc.
Conventionally, barium titanate powder was prepared by reacting titanium oxide with barium carbonate in a solid phase. According to the recent trend toward small-size/large capacitance (by high dielectric constant compositions, and thin- and multi-layering of dielectrics), low-temperature sintering, high frequency and high performance MLCCs, barium titanate powder has been required to be of high purity/composition homogeneity, fine particle/particle size uniformity, and non-coagulation/high dispersibility. To satisfy the requirements, liquid phase synthesis of barium titanate was suggested, which has been developed to hydrothermal synthesis, co-precipitation (oxalate synthesis) and alkoxide synthesis (sol-gel synthesis).
According to the oxalate synthesis, Ba and Ti ions are precipitated to barium titanyl oxalate (BTO) in the presence of oxalate in a solution and the precipitate is dried and thermally decomposed to barium titanate powder.
Referring to FIG. 1, there is illustrated a conventional method for preparing barium titanate powder by oxalate synthesis. As seen in FIG. 1, an aqueous barium chloride solution is mixed with an aqueous titanium chloride solution at a Ba/Ti molar ratio of 1:1 and added with oxalic acid to give barium titanyl oxalate (BTO) (BaTiO(C2O4)2.4H2O) as a precipitate which is then washed, dried and filtered, followed by thermolysis at 800xc2x0 C. or higher to produce barium titanate powder.
Having the advantage of being simple and requiring low material cost and low investment for facilities, the oxalate synthesis process was first commercialized.
However, the oxalate synthesis process is disadvantageous in that it is difficult to control the powder composition (Ba/Ti) and particle distribution. Also, the particles have a strong tendency to aggregate upon thermolysis, resulting in an incomplete pulverization, after which there may remain aggregates as large as ones to tens of xcexcm in size. Further, the particles are not well dispersed upon subsequent mixing/forming processes. In addition, the presence of aggregates may result in the formation of abnormal crystal grains at sintering.
Another disadvantage of the conventional oxalate synthesis process is that the powder is not suitable for use in MLCC B characteristics because the grains cannot be grown and shows poor crystallinity owing to the presence of aggregates.
In more recent times, the barium titanate powder prepared in the hydrothermal synthesis process has been substituted for that prepared in the oxalate synthesis process to cope with the requirement for thinning and multiple layering of MLCCs. Despite excellent properties, the barium titanate powder prepared in the hydrothermal synthesis process is not extensively used because the synthesis process is complex and requires the use of autoclaves, leading to poor productivity and expensive product.
Therefore, there remains a need for a method for preparing barium titanate powder at low cost, and with ease.
It is an object of the present invention to provide a method for preparing barium titanate powder of excellent powder properties by oxalate synthesis, which is of high productivity with high economic advantage.
It is another object of the present invention to provide a method for preparing doped-barium titanate powder of excellent powder properties by oxalate synthesis, which is of high productivity with high economic advantage.
In accordance with an aspect of the present invention, there is provided a method for preparing barium titanate by oxalate synthesis, comprising the steps of: adding an aqueous barium chloride solution and an aqueous titanium chloride solution to an aqueous oxalic acid solution to give barium titanyl oxalate as a precipitate, aging, washing, filtering and drying the precipitate; primarily calcining the barium titanyl oxalate, followed by primary pulverization to afford fine barium titanyl powder; and secondarily calcining the fine barium titanyl powder, followed by secondary pulverization.
In accordance with another aspect of the present invention, there is provided a method for preparing barium titanate by oxalate synthesis, comprising the steps of: dissolving at least one donor or acceptor in a mixture of an aqueous barium chloride solution and an aqueous titanium chloride solution to give a dopant element-containing solution, said donor or acceptor being in a form of oxides, hydroxides, carbonates, nitrides, sulfates or salts, and being selected from the group consisting of Y, La, V, Nb, Ta, Cr, Mo, W, Mn, Fe, Co, Rh, Ni, Cu, Zn, Ga and rare-earth elements; adding the dopant element-containing solution to an aqueous oxalic acid solution to give doped-barium titanyl oxalate as a precipitate, aging, washing, filtering and drying the precipitate; primarily calcining the doped-barium titanyl oxalate, followed by primary pulverization to afford fine doped-barium titanyl powder; and secondarily calcining the doped-fine barium titanyl powder, followed by secondary pulverization.